The ferry ride to Peaks Island takes only about 20 minutes, so it is the perfect choice for a short day trip. Once you land, you can visit the shops for a short while and perhaps have lunch or a beverage at one of the restaurants nearby before taking the ferry back to the mainland. The ferry seems to leave every hour. You might also extend your visit by doing some walking around the small island; it is only about 4 miles around the Peaks Island Loop.
Rather than repeat the information available from Casco Bay Lines’ web site, I encourage you to check out their website at the following link – Casco Bay Lines – Peaks Island.